Solid Electrolytic Capacitor

ABSTRACT

An outer insulation member made of a first resin packs a part of an anode terminal. The packed part of the anode terminal has a predetermined section. All surfaces of the predetermined section consist of a front surface, a back surface and edges connecting between the front surface and the back surface. The front surface includes a connection area. An anode lead wire is connected to the connection area while being not connected to the anode terminal other than the connection area. The all surfaces of the predetermined section are completely covered with a mask layer made of a second resin except for the connection area. The second resin is different from the first resin in at least one of composition thereof, content of an inclusion, size of an inclusion and shape of an inclusion.

BACKGROUND OF THE INVENTION

This invention relates to a solid electrolytic capacitor that has highhumidity resistance.

For example, a solid electrolytic capacitor is disclosed inJP2000-208367A, which is incorporated herein by reference. The disclosedsolid electrolytic capacitor includes an anode body coated with adielectric film, a solid electrolyte layer formed on the dielectricfilm, a cathode layer formed on the solid electrolyte layer, a cathodelead, an anode lead wire attached to or partially embedded in the anodebody, an anode terminal connected to the anode lead wire, and an outerinsulation member encasing all but a portion of the anode and cathodeleads. The anode lead wire and the anode terminal may be connected toeach other before the formation of the solid electrolyte layer. Theconnection portion between the anode lead wire and the anode terminal iscovered with and fixed by resin, which is formed in a process differentfrom the formation process of the outer insulation member. According toJP2000-208367A, since the anode lead wire and the anode terminal areconnected before the formation of the solid electrolyte layer, the solidelectrolyte layer can be prevented from being damaged by stress upon theconnection between the anode lead wire and the anode terminal.

The solid electrolytic capacitor of JP2000-208367A as well as otherconventional solid electrolytic capacitors might be broken down whenused under high humidity atmosphere.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a solidelectrolytic capacitor that has high humidity resistance.

As a result of humidity test for solid electrolytic capacitor, thepresent inventors have found out that one cause of propertydeterioration of a solid electrolytic capacitor is metal ions elutingfrom an anode terminal. For example, an anode terminal is formed of acopper plate plated with nickel and/or tin. Contact of water on such ananode terminal generates metal ions, which might migrate towards acathode layer together with penetration of water. If the metal ionsreach a solid electrolyte layer and a dielectric film of a capacitorelement, equivalent series resistance (ESR) of the solid electrolytelayer becomes large, and leak current through the dielectric filmincreases. Therefore, it is preferable that the anode terminal beprevented from being in contact with water. In particular, if metal ionselute from a section of the anode terminal close to the capacitorelement, the above-described problems will happen easily because anassumed path length of migration of metal ions is short. The presentinventors have identified, as a predetermined section, a sectionrelating to the generation of problematic metal ions and have found thatthe outer insulation member is not sufficient to prevent generation ofmetal ions from the predetermined section. Therefore, the presentinventors have considered that the predetermined section isintentionally covered with a resin prior to the formation process of theouter insulation member. Based on the above findings and consideration,the present invention provides an improved solid electrolytic capacitoras further described herein.

One aspect of the present invention provides a solid electrolyticcapacitor which comprises a capacitor element, a cathode terminal, anouter insulation member and an anode terminal. The capacitor elementincludes an anode body, an anode lead wire and a cathode layer. Theanode lead wire extends from the anode body. The cathode terminal isconnected to the cathode layer. The outer insulation member is made of afirst resin. The outer insulation member encases the capacitor elementand a part of the cathode terminal. A part of the anode terminal is alsoencased by the outer insulation member. The part of the anode terminalhas a predetermined section, all surfaces of the predetermined sectionconsisting of a front surface, a back surface and edges connectingbetween the front surface and the back surface. The front surfaceincludes a connection area. The anode lead wire is connected to theconnection area while the anode lead wire is not connected to the anodeterminal other than the connection area. All surfaces of thepredetermined section are completely covered with a mask layer made of asecond resin except for the connection area. The first resin and thesecond resin are different from each other in at least one ofcomposition thereof, contents of inclusions, sizes of inclusions andshapes of inclusions.

In the predetermined section, areas other than the connection area arecompletely covered with the mask layer made of the second resin, whilethe connection area is hid by the connection with the anode lead wire soas not to be exposed. The outer insulation member is made of the firstresin. The first resin and the second resin are different from eachother in at least one of composition thereof, contents of inclusions,sizes of inclusions and shapes of inclusions. In short, thepredetermined section is intentionally covered with the second resin ofthe mask layer which is distinguishable from the first resin of theouter insulation member. Therefore, generation of metal ions from thepredetermined section can be suppressed so that property deteriorationof a solid electrolytic capacitor due to metal ion migration can bereduced.

An appreciation of the objectives of the present invention and a morecomplete understanding of its structure may be had by studying thefollowing description of the preferred embodiment and by referring tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a solid electrolytic capacitorin accordance with an embodiment of the present invention.

FIG. 2 is a transparent perspective view showing parts of the solidelectrolytic capacitor of FIG. 1.

FIG. 3 is a cross-sectional view showing a solid electrolytic capacitorof an embodiment of the invention.

FIG. 4 is a perspective view showing an anode terminal and an anode leadwire of the solid electrolytic capacitor of FIG. 3 shown in isolatedview.

FIG. 5 is a cross-sectional view showing a solid electrolytic capacitorof an embodiment.

FIG. 6 is a cross-sectional view showing a solid electrolytic capacitorof an embodiment.

FIG. 7 is a cross-sectional view showing a solid electrolytic capacitorof an embodiment.

FIG. 8 is a cross-sectional view showing a solid electrolytic capacitorof an embodiment.

FIG. 9 is a cross-sectional view showing a solid electrolytic capacitorof an embodiment.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all embodiments, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims.

DETAILED DESCRIPTION

The present invention is related to an improved capacitor. Morespecifically, the present invention is related to an improved capacitorwith a portion of the anode lead encased to prohibit migration of metalparticles.

The invention will be described with reference to the figures which arean integral, but non-limiting part of the specification. Throughout thevarious figures similar elements will be numbered accordingly.

With reference to FIGS. 1 and 2, a solid electrolytic capacitor 1according to an embodiment of the present invention comprises acapacitor element 10, an anode terminal 30, a cathode terminal 50, anouter insulation member 60 made of a first resin and a mask layer 70made of a second resin, wherein the mask layer 70 partially covers theanode terminal 30. As shown in FIG. 1, the capacitor element 10comprises an anode body 12 with a dielectric 16 thereon, an anode leadwire 14, a solid electrolyte layer 18, and a conductive layer 20preferably comprising at least one of a graphite layer and a silverpaste layer. The conductive layer allows for adhesion to the solidelectrolyte layer which is otherwise difficult to attach a cathode leadthereto. In use the anode terminal, 30, is in electrical contact with ananode trace 80 of a circuit board 82 and the cathode is in electricalcontact with a cathode trace 84 of the circuit board.

The anode body 12 of the present embodiment is preferably formed of avalve metal or a conductive oxide of a valve metal. Sintered tantalumpowder is suitable for demonstration of the invention. On the surface ofthe anode body 12, a dielectric film 16 is formed. The anode lead wire14 extends from the anode body 12. The anode wire may be the samecomposition as the anode, which is preferred, or another conductivematerial. A tantalum wire is suitable for demonstration of the inventionwhen the anode is tantalum. In one embodiment, the anode lead wire 14 ispartially embedded in the anode powder, and the anode powder is thenpress-molded so that a molded member is obtained. Subsequently, themolded member is sintered, such as at 1200° C., so that the anode body12 of the sintered anode power is obtained. Thereafter, the anode body12 of the sintered anode power is soaked into an aqueous solution suchas a phosphoric acid solution to be anodized so that an anodic oxidefilm, or the dielectric film, is formed on the surface of the anode body12 and, in some embodiments, the surface of the anode lead wire 14.

The anode body 12 is alternately soaked into a liquid of thiophene andan oxidizer so that a chemical polymerization is repeatedly carried outto form the solid electrolyte layer 18 of polythiophene on thedielectric film 16. Namely, the solid electrolyte layer 18 of thepresent embodiment is made of conductive polymer. Other polymers such aspolyaniline and polypyrrole are suitable for demonstration of theinvention with polythiophene, and particularly poly(ethylene3,4-dioxythiophene) as described in U.S. Pat. No. 7,667,954 which isincorporated herein by reference, being preferred. A conductive layer 20is preferably formed to improved adhesion to the solid electrolytelayer. The conductive layer 20 preferably comprises a graphite layerformed on the solid electrolyte layer and a silver paste layer formed onthe graphite layer so that the capacitor element 10 is obtained. Inother words, a cathode layer of the capacitor element 10 of the presentembodiment is made of the solid electrolyte layer 18 and the conductivelayer wherein the conductive layer may comprise a graphite layer and thesilver paste layer. However, the cathode layer of the capacitor element10 is not limited thereto. In some embodiments the silver paste layermay be omitted.

The composition of the anode terminal 30 and the cathode terminal 50 isnot particularly limited herein. A base member of copper frame platedwith nickel and tin is particularly suitable for demonstration of theinvention. A nickel plated layer having a thickness of about 1 μm formedon a copper frame base member having a thickness of about 30 μm, and atin plated layer having a thickness of about 2 μm formed on the nickelplated layer is particularly suitable as an anode or cathode terminalfor demonstration of the invention. Thus, the anode terminal 30 and thecathode terminal 50 are obtained. The anode lead wire 14 is preferablywelded to the anode terminal 30 such as through resistance welding orother techniques known in the art so that the anode lead wire 14 isconnected to a connection area 42 of the anode terminal 30. The anodelead wire 14 is not connected to areas other than the connection area42. The cathode terminal 50 is preferably bonded to the conductive layer20 by using a conductive adhesive agent. The conductive adhesive agentmade include silver fillers and epoxy resin or other conductiveadhesives and related layers as known in the art.

The second resin is applied to a part of the anode terminal 30 to formthe mask layer 70. In one embodiment, the second resin of the presentembodiment is made of silicone. Namely, the mask layer 70 of the presentembodiment comprises silicone without limit thereto. The second resinmay include, as its base, fluoroplastic, epoxy resin, acrylic resin,propylene resin or ester resin. The second resin may be formed by mixinga water repellent and a paint made of any resin and may include a waterrepellent as an inclusion.

The mask layer 70 of the present embodiment preferably has a contactangle of water droplets of not smaller than 80 degrees in order tosuppress problematic metal ion generation. The contact angle of waterdroplets can be measured via a method defined by ISO 15989. In view ofreliable prevention of problematic metal ion generation, it ispreferable that the mask layer 70 has a contact angle of water dropletsof not smaller than 85 degrees. In order to ensure a water repellentability of the mask layer 70, it is further preferable that the masklayer 70 has a contact angle of water droplets not smaller than 90degrees. It is still further preferable that the mask layer 70 has notonly water-repellent function but also waterproof function and waterresistant function.

In the present embodiment, an area where the second resin is applied, oran area where the mask layer 70 is formed, is a first portion 32 of theanode terminal 30 which may occupy one third of the whole region of theanode terminal 30. However, since the anode lead wire 14 is connected tothe connection area 42, the connection area 42 is not applied with thesecond resin. Namely, the mask layer 70 does not directly cover theconnection area 42. Thus, the second resin is applied to the entirefirst portion 32 other than the connection area 42. In other words, thefirst portion 32 is completely covered with the mask layer 70 and theanode lead wire 14.

The outer insulation member 60 is formed so as to enclose a part of theanode terminal 30 and a part of the cathode terminal 50 and encase theentire capacitor element 10. In one embodiment the outer insulationmember 60 is made of epoxy resin formed by injection molding with ametal mold of a predetermined shape, followed by hardening it. The firstresin and the second resin are different from each other in at least oneof composition, contents of inclusions, sizes of inclusions and shapesof inclusions. For example, the first resin of the present embodimentmay include a silica filler as an inclusion in order to lower thermalcoefficient of expansion of the outer insulation member 60 and isdifferent from the second resin of the present embodiment. Therefore,the first resin and the second resin are distinguishable from eachother. For example, a boundary between the mask layer 70 and the outerinsulation member 60 can be identified by scanning electron microscopy(SEM).

The thus-formed outer insulation member 60 encloses the part of theanode terminal, which is the first portion 32. A bottom of the outerinsulation member 60 may be fixed to a circuit board when the solidelectrolytic capacitor 1 is mounted on the circuit board. Namely, thebottom of the outer insulation member 60 can be a mount surface of thesolid electrolytic capacitor 1. After the formation of the outerinsulation member 60, each of the anode terminal 30 and the cathodeterminal 50 is folded to the bottom of the outer insulation member 60 tohave an angular C-shape. Thus, the solid electrolytic capacitor 1 isobtained. The anode terminal 30 of the angular C-shape has a secondportion 44 and a coupling portion 46 in addition to the aforementionedfirst portion 32. The first portion 32 is covered by the outerinsulation member 60. The second portion 44 may be exposed on the mountsurface of the solid electrolytic capacitor 1, or the bottom of theouter insulation member 60. The coupling portion 46 couples the firstportion 32 and the second portion 44 with each other and may be exposedon the side of the outer insulation member 60.

As shown in FIG. 2, the first portion 32 includes a predeterminedsection 34, as described afterwards. The predetermined section 34includes the connection area 42. In detail, all surfaces of thepredetermined section 34 consists of a front surface 36, a back surface38 and all edges 40 connecting between the front surface 36 the backsurface 38. The connection area 42 is located within or on the frontsurface 36. As apparent from FIG. 2, all surfaces of the predeterminedsection 34 are completely covered with the mask layer 70 except for theconnection area 42. Specifically, as shown in FIG. 2, the first portion32 is completely covered with the mask layer 70 except for theconnection area 42. Therefore, it can be prevented that water contactsonto the first portion 32 and therefore can not cause metal ions to beeluted from the first portion 32 or to move towards the capacitorelement 10.

Thus, the solid electrolytic capacitor 1 of the present embodiment canprevent metal ions from migrating from the anode terminal 30 to thesolid electrolyte layer 16 of the capacitor element 10.

Although the first portion 32 is preferably wholly applied with thesecond resin to form the mask layer 70 in the above-describedembodiment, the present invention is not limited thereto. Provided thatthe mask layer 70 of the second resin completely covers all surfaces ofthe predetermined section 34 except for the connection area 42, the masklayer 70 may cover other sections.

A solid electrolytic capacitor 1 a is illustrated in FIGS. 3 and 4. InFIGS. 3 and 4, a second resin is not applied for the whole first portion32 but only for the predetermined section 34. Specifically, in the firstmodification, all surfaces of the predetermined section 34, i.e., thefront surface 36, the back surface 38 and all edges 40 relating to thepredetermined section 34, are completely covered with the mask layer 70a except for the connection area 42. Thus, parts of the first portion 32close to the capacitor element 10 are covered with the mask layer 70 a.Therefore, it can be prevented that water contacts on the predeterminedsection 34 and, accordingly, causes metal ions to be eluted from thepredetermined section 34 and to move towards the capacitor element 10.

In FIG. 5, a second resin is wholly applied for the first portion 32 andthe coupling portion 46. Specifically, all surfaces of the first portion32 and the coupling portion 46, which include those edges as a matter ofcourse, are completely covered with the mask layer 70 b except for theconnection area 42. Therefore, it can be prevented that water contactson the first portion 32 thereby inhibiting metal ions from being elutedfrom the first portion 32 and moving towards the capacitor element 10.In addition, since the coupling portion 46 is positioned outside of theouter insulation member 60 but is protected by the mask layer 70 b metalion elution from the coupling portion 46 is inhibited. Although theentire coupling portion 46 is covered with the mask layer 70 b in thesecond modification, the present invention is not limited thereto. Ifthe coupling portion 46 is, at least in part, covered with the masklayer 70 b, results can be obtained in correspondence with the coveredareas.

In FIG. 6, a second resin is applied for the whole first portion 32 andfor a part of the anode lead wire 14. Specifically, the second resin isapplied so as to completely cover a part of the anode lead wire 14 closeto the connection area 42. Thus, a connection portion between the anodelead wire 14 and the anode terminal 30 is completely covered, preferablycompletely, with the mask layer 70 c. Therefore, it can be preventedthat metal ions are eluted from the first portion 32. In addition, evenif metal ions are eluted, movement of the metal ions on the anode leadwire 14 can be reduced.

In FIG. 7, a second resin is applied for the entire first portion 32 andfor the entire anode lead wire 14. Thus, the anode lead wire 14 and thefirst portion 32 are completely covered with the mask layer 70 d.Therefore, it can be prevented that metal ions are eluted from the firstportion 32. In addition, even if metal ions are eluted, the eluted metalions can be prevented from moving on the anode lead wire 14.

In FIG. 8, a second resin of a mask layer 70 e is wholly applied for thefirst portion 32 and the coupling portion 46 and is also applied for theentire anode lead wire 14.

A solid electrolytic capacitor 1 f is shown in FIG. 9 wherein a secondresin of a mask layer 70 f is applied for the entire capacitor element10 and for parts of the anode terminal 30 and the cathode terminal 50enclosed by the outer insulation member 60. Specifically, the secondresin of the mask layer 70 f is wholly applied for the first portion 32of the anode terminal 30 and is wholly applied for a part of the cathodeterminal 50, which corresponds to the first portion 32. If portionsother than the predetermined section 34 are, at least in part, coveredwith the mask layer 70 f, results can be obtained in correspondence withthe covered areas. However, if the mask layer 70 f is formed so as toenclose a part of the anode terminal 30 and a part of the cathodeterminal 50 and to completely package the capacitor element 10 as shownin FIG. 9, an elution source of metal ions can be completely shut down.In addition, even if metal ions are eluted, movement paths of the elutedmetal ions to the capacitor element 10 are completely blocked.

Although each of the mask layers 70 to 70 f of the above-describedembodiments are formed on the predetermined section 34 and so on byapplication of the second resins, the present invention is not limitedthereto. Methods other than application of a second resin may be used toform a mask layer on the predetermined section 34 and so on. Forexample, in the case of the solid electrolytic capacitor if thecapacitor element 10 and so on may be dipped into the second resin sothat the mask layer 70 f encloses a part of the anode terminal 30 and apart of the cathode terminal 50 and completely encases the capacitorelement 10. However, in case of the dipping process, the entire cathodeterminal 50 is temporarily covered with the second resin, the undesiredsection of second resin must be removed by sand-blasting or wet-blastingin a subsequent process. Therefore, it is preferable that the mask layeris formed by application of the second resin in order to prevent thenumber of processes from increasing. In one embodiment the mask layermay be formed by other methods such as spraying or coating.

The anode is a conductor and preferably a valve metal or conductiveoxide of a valve metal. Particularly preferred valve metals include Al,W, Ta, Nb, Ti, Zr and Hf. Most preferably, the anode comprises at leastone material selected from the group consisting of Al, Ta, Nb and NbOwith tantalum being most preferred.

The anode wire is a conductor and preferably a valve metal or conductiveoxide of a valve metal. Particularly preferred valve metals include Al,W, Ta, Nb, Ti, Zr and Hf. Most preferably, the anode comprises at leastone material selected from the group consisting of Al, Ta, Nb and NbOwith tantalum being most preferred. In one embodiment the anode wire hasthe same composition as the anode.

While there has been described what is believed to be the preferredembodiment of the invention, those skilled in the art will recognizethat other and further modifications may be made thereto withoutdeparting from the spirit of the invention, and it is intended to claimall such embodiments that fall within the true scope of the invention.

What is claimed is:
 1. A solid electrolytic capacitor comprising: acapacitor element including an anode body, an anode lead wire extendingfrom said anode body and a cathode layer; a cathode terminal connectedto said cathode layer; an outer insulation member comprising a firstresin, wherein said outer insulation member encases said capacitorelement and a portion of said cathode terminal; and an anode terminal, aportion of which is encased in said outer insulation member, whereinsaid anode terminal has a predetermined section, all surfaces of thepredetermined section consisting of a front surface, a back surface andedges connecting between said front surface and said back surfacewherein said front surface includes a connection area with said anodelead wire being connected at said connection area while being notconnected to said anode terminal other than at said connection area,wherein all surfaces of said predetermined section except for saidconnection area are covered with a mask layer comprising a second resin,said first resin and said second resin being different from each otherin at least one of composition, contents of inclusions, sizes ofinclusions and shapes of inclusions.
 2. The solid electrolytic capacitorof claim 1, wherein: said mask layer has a contact angle of waterdroplets not smaller than 80 degrees.
 3. The solid electrolyticcapacitor of claim 2, wherein: said mask layer has a contact angle ofwater droplets not smaller than 85 degrees.
 4. The solid electrolyticcapacitor of claim 3, wherein: said mask layer has a contact angle ofwater droplets not smaller than 90 degrees.
 5. The solid electrolyticcapacitor of claim 1, wherein: said anode terminal includes a firstportion, a second portion and a coupling portion coupling said firstportion and said second portion; said second portion is fixed to acircuit board when said solid electrolytic capacitor is mounted on saidcircuit board; said first portion is provided with said predeterminedsection; and said mask layer completely covers said first portion exceptfor said connection area.
 6. The solid electrolytic capacitor of claim5, wherein said mask layer covers, at least in part, said couplingportion.
 7. The solid electrolytic capacitor of claim 1, wherein saidmask layer covers at least a portion of said anode lead wire.
 8. Thesolid electrolytic capacitor as recited in claim 1, wherein said masklayer covers at least a portion of said capacitor element, part of saidanode terminal and part of said cathode terminal.
 9. The solidelectrolytic capacitor as recited in claim 1, wherein: said anode bodyand said anode lead wire are made of tantalum; and said anode terminalis formed of a copper plate plated with at least one of nickel and tin.10. The solid electrolytic capacitor as recited in claim 1, wherein saidsecond resin is made of silicone or fluoroplastic.
 11. The solidelectrolytic capacitor as recited in claim 1, wherein said capacitorelement further includes a solid electrolyte layer made of conductivepolymer.
 12. A method for forming a capacitor comprising: providing ananode with a dielectric thereon and an anode lead wire extendingtherefrom; providing an anode terminal comprising a first portion, asecond portion and a coupling portion; attaching said anode lead wire tosaid first portion at a connection area; encasing at least a portion ofsaid anode terminal with a mask layer comprising a second resin;encasing said capacitor with a first resin wherein said first resin andsaid second resin differ by at least one of composition, contents ofinclusions, sizes of inclusions and shapes of inclusions.
 13. The methodfor forming capacitor of claim 12, wherein: said mask layer has acontact angle of water droplets not smaller than 80 degrees.
 14. Themethod for forming capacitor of claim 13, wherein: said mask layer has acontact angle of water droplets not smaller than 85 degrees.
 15. Themethod for forming capacitor of claim 14, wherein: said mask layer has acontact angle of water droplets not smaller than 90 degrees.
 16. Themethod for forming capacitor of claim 12, wherein: said anode terminalincludes a first portion, a second portion and a coupling portioncoupling said first portion and said second portion; fixing said secondportion to a circuit board; said first portion is provided with saidpredetermined section; and said mask layer completely covers said firstportion except for said connection area.
 17. The method for formingcapacitor of claim 16 further comprising covering at least in part ofsaid coupling portion with said mask layer.
 18. The method for formingcapacitor of claim 12, further comprising covering at least a portion ofsaid anode lead wire with said mask layer.
 19. The method for formingcapacitor as recited in claim 12, wherein said mask layer covers atleast a portion of said capacitor element, part of said anode terminaland part of said cathode terminal.
 20. The method for forming capacitoras recited in claim 12, wherein: said anode body and said anode leadwire are made of tantalum; and said anode terminal is formed of a copperplate plated with at least one of nickel and tin.
 21. The method forforming capacitor as recited in claim 12, wherein said second resin ismade of silicone or fluoroplastic.
 22. The method for forming capacitoras recited in claim 12, wherein said capacitor element further includesa solid electrolyte layer made of conductive polymer.